RogueRose - 9-5-2016 at 17:54
I made some Tumeric pH testing strips (paper towels & q-tips) that can be used long after being prepared. I'm wondering if the same can be done
with red cabbage juice? I have a cabbage head and IDK if I can make the juice and either freeze some of it or make strips like the tumeric.
Does anyone know if that can be done with paper towels and q-tips or if freezing it will hurt it?
[Edited on 10-5-2016 by RogueRose]
j_sum1 - 9-5-2016 at 18:17
I have seen it grow mould and smell bad after only a few weeks. I think that freezing would work ok. Or you could freeze actual cabbage and extract
when needed. You could try dehydrating to a powder. You might have some success in using a solvent other than water -- one prohibitive to organisms.
Methanol might be a good one to try. Or you could add a preservative of some kind.
But really, I would just get a new cabbage when I needed one, or make do with commercially available indicators and pH papers.
Turmeric is a great indicator for boron-containing compounds. 
RogueRose - 9-5-2016 at 19:34
Thanks for the suggestions. I tried extracting the juice from chopped cabbage using denatured alcohol (IDK if it is mainly ethanol or methanol..) and
then heated in microwave at 20% heat until about 140 degrees. I then pressed cabbage against side of container to remove liquid.
For the extraction in water I used DH2O. Chopped cabbage was added to a beaker and covered with water and heated until boiling. I used a food
processor to puree then pressed through a strainer & liquid through coffee filter.
The alcohol did a much superior job to extracting the color as the cabbage was a light purple as opposed to the mid purple of the water extraction.
I'm going to try to reduce the liquid volume but am worried about heat effecting the substance that identifies the pH.
j_sum1 - 9-5-2016 at 21:06
You could simply juice the cabbage. I don't think heat is needed to remove the pigment from the cells.
JJay - 10-5-2016 at 15:58
I've boiled red cabbages for pH indicator a few times. Heat won't hurt it much. I never tried storing the indicator for more than a few weeks in the
refrigerator, though.
Ozone - 10-5-2016 at 16:21
It also tends to oxidize and polymerize in air (sinking brown stuff). So, to keep it for any time, I'd go as free of air as possible, in the dark, and
cold. If you freeze it, be prepared to filter out the stuff that precipitates (less soluble polysaccharide, protein, etc.) before use (or so I've
noticed). They also really hate (actually, really like) iron +3 (and the REDOX cycling with Fe2+).
It's really better to make this as-needed. Also, elderberries (and stillage from elderberry wine) are good, too. But, like other anthocyanidins and
other bioflavonoids, suffer from similar issues with stability.
O3
ave369 - 22-5-2016 at 01:50
In my experience, hibiscus herbal tea (marketed as "Red Zinger", "Italian Tea", etc) can be used as a better alternative to cabbage. From it and
alcohol, you can get better, longer-lasting paper strips.
MeshPL - 24-5-2016 at 11:26
I once made a cabbage juice indicator. It fermented after a few days. Nice thing is that it changes color once it gets stale. (Turns red from
bluish-purple)
I guess you could store your extract in room temperature for much longer if you pasteurized it. My grandma makes (concentrated) juices this way and
they are good even after a year or so. You can also try adding a bunch of salt to your indicator, which would act as a natural preservative. Unless
you need your indicator salt-free.
What also works as an indicator is carrot(very weak), purple eggplant skin (simmilar to cabbage), red onion(good, but fewer transitions than cabbage),
black grapes(they smell nice, simmilar to caaabbage), and other red fruit(weak). The one which spoils the most slowly is onion, judging from my
experience.
However the cabbage seems to have the richest colours: red, violet, blue, green, yellow. Orange is the only lacking one.
Sulaiman - 24-5-2016 at 11:41
I did not realise what a range of pH is covered !
colour vs. pH charts for red cabbage juice
https://www.google.co.uk/search?q=colour+vs.+pH+chart+for+re...
on average, does the concentration required for easy pH determination with red cabbage juice
significantly interfere with titration or end-point determination etc. ?
[Edited on 24-5-2016 by Sulaiman]
Amos - 24-5-2016 at 12:15
Extract the anthocyanins from red cabbage with methanol or another alcohol instead of water. As long as this extract is kept neutral or mildly acidic
(violet to red color range) in an airtight container, it will keep for a very long time and can still be used as indicator at any time.
[Edited on 5-24-2016 by Amos]
pantone159 - 24-5-2016 at 13:08
I have had good luck extracting with 95% EtOH. I think I use about 50 g of 95% EtOH, and 50 g of red cabbage, and blend. This definitely keeps better
than a water extract, though I do not remember how long. I did not know to try and make sure it is not basic, next time I will add a drop or so of
acid to the extract after I prepare it.
ave369 - 24-5-2016 at 14:10
Our wiki has an article for anthoocyanin. http://www.sciencemadness.org/smwiki/index.php/Anthocyanin
It even has the color chart!
ahill - 24-5-2016 at 15:16
I kept mine in an airtight jar with about 30% isopropyl alcohol.
I used it intermittently over 4 or 5 months during which time it slowly became browner and less sensitive until it was effectively useless.
I also used it to make 'litmus' paper by soaking filter paper in it then drying it. The filter paper is still good (purple-pink and quite sensitive)
after a couple of years !
Eventually I bought an electronic pH meter, and while I use the paper strips on occasion, I just dont bother with the solution any more, and am not
tempted to make up any more.
So while its an interesting project to make, and suprisingly practical - I highly recommend electric pH meters - they are quite cheap and very
practical.